Researchers have worked for years to increase the storage density and reduce the cost per bit of computer system storage devices. These efforts have met with some success, as storage density has increased and cost per bit has dropped in conventional storage devices, such as magnetic hard-drives, optical drives and dynamic random access memory (DRAM). However, it has become increasingly difficult to put more information into storage devices using conventional technologies, which may be approaching fundamental limits.
Scientists have proposed alternative approaches in an attempt to increase storage density and reduce cost per bit. In one approach, referred to as Scanned Probe Microscopy (SPM), a probe is positioned extremely close to a storage medium. In one configuration, referred to as Scanning Tunneling Microscopy (STM), the probe is positioned within a few nanometers of the storage medium. Positioning the probe close to the storage medium ensures the probe is within tunneling range of the medium. However, precisely controlling the spacing between the probe and storage medium is a difficult and expensive task. In another configuration, referred to as Atomic Force Microscopy (AFM), the probe actually touches the medium. In each of these configurations, it is difficult and expensive to build a storage system where the storage medium and/or probe are not eventually damaged.
Some researchers have tried approaches that eliminate the need for extremely close proximity or contact between the probe and storage medium. Some approaches are based on non-contact Scanning Force Microscopy (SFM), which typically suffers from poor resolution and poor signal to noise ratio. Another approach is based on Near-Field Scanning Optical Microscopy, which has limited lateral resolution and slow access times. The utility of the storage device is limited if it takes a long time to retrieve stored information. High storage density, low cost per bit and fast access times are needed in a computer system storage device.
In one embodiment, the storage medium is a heterojunction diode including a phase change material for information storage. Field emitters write information into a storage area of the storage medium by emitting an electron beam into the phase change material. The magnitude of the electron beam is increased and decreased to change the state of the storage area on which it impinges. Information is read from the storage device by bombarding a storage area with an electron beam to generate a signal current from the storage area. The magnitude of the signal current depends on the state of the storage area. The information stored in the storage area (i.e., the state of the storage area) can be determined from the magnitude of the signal current collected through the storage medium.